1. Field of the Invention
Embodiments of the present invention relate generally to optical spectroscopy. More particularly, embodiments of the present invention relate to depth-resolved measurements of fluorescence and reflectance properties of a specimen.
2. Description of Related Art
Optical spectroscopy is emerging as an effective diagnostic technique for noninvasive detection of cancers and prefacers that originate in the epithelial lining of organs such as the uterine cervix, the oral cavity, the urinary bladder, and the esophagus. The progression of precancer in these tissues produces morphologic and biochemical changes in the epithelium and supporting stroma. These changes include alterations in epithelial cell morphology and metabolic activity, changes in stromal protein morphology and cross-linking, and increasing stromal angiogenesis. As a result, the concentration and distribution of endogenous fluorophores such as reduced nicotinamideadenine dinucleotide, flavin adenine dinucleotide, keratin, tryptophan, and collagen cross-links, and absorbers such as hemoglobin, are altered with the progression of precancer. Thus, knowledge of the depth-dependent distribution of chromophores may have important diagnostic significance.
Endogenous chromophores can be detected noninvasively in vivo by use of fiber-optic fluorescence and reflectance spectroscopy. Many fiber-optic probe designs collect the integrated signal from both the epithelium (which is typically of the order of 300 μm thick) and the underlying stroma. In these systems, sophisticated analysis strategies are required for deconvolution of spectroscopic data to yield quantitative concentrations of chromophores, and little information about depth-related changes is obtained. Fiber-optic probes that can localize spectroscopic information by depth to distinguish epithelial and stromal optical signatures should improve the ability of spectroscopy to evaluate noninvasively the progression of precancerous changes.
A variety of probe designs for obtaining localized or depth-resolved spectroscopic data have been reported. Single-fiber probe configurations, in which the same fiber is used for illumination and collection, are sensitive to light scattering from superficial tissue regions. However, the use of single-fiber probes for optical measurements is limited by lower signal-to-noise ratios that are due to autofluorescence generated by impurities in the fiber core and by specular reflection from fiber surfaces. With multiple-fiber probes, many configurations are possible. Straight-fiber geometries with different source detector separations permit some depth discrimination; however, in epithelial tissue the signal from the stroma tends to dominate, even at minimum source-detector separation. Angled illumination and collection fibers can be used to target specific depth regions. Targeting the epithelial layer, however, requires steep angles that may increase the diameter of the probe so as to be impractical for clinical probe designs.
It is therefore desirable to obtain depth-resolved spectroscopic data without the associated limitations noted in previous designs. Referenced shortcomings of conventional methodologies and apparatus mentioned above are not intended to be exhaustive, but rather are among several that tend to impair the effectiveness of previously known techniques concerning optical spectroscopy. Other noteworthy problems may also exist; however, those mentioned here are sufficient to demonstrate that methodology and apparatus appearing in the art have not been altogether satisfactory and that a significant need exists for the techniques and apparatus described and claimed here.